1. Field of the Invention
This invention relates to a V-ribbed belt having bottom and back faces each provided with the same number of ribs and relates to a method for fabricating the V-ribbed belt.
2. Description of the Prior Art
As an example of V-ribbed belts of this type, there is conventionally known one in which the bottom and back faces of the belt each have the same number of ribs of approximately trapezoidal section formed in corresponding positional relation between the bottom and back faces, for example, as disclosed in U.S. Pat. Nos. 5,273,496, 5,334,107 and 5,507,699.
More specifically, if the top of each rib has an arcuate section, right and left side faces of each rib will act as contact portions in contact under pressure with side faces of a pulley groove of a V-ribbed pulley to contribute to power transmission. In this case, however, the top of each rib not in contact with any surface of the pulley groove forms an unnecessary portion not required for power transmission, and the unnecessary portion induces a problem that it causes cracks due to bending of the belt. Therefore, in the above conventional example, the unnecessary portion at the rib top is eliminated by forming the rib into an approximate trapezoidal section.
Meanwhile, this double-sided V-ribbed belt may be used in a normal position in which the bottom face of the belt is used as an inner peripheral side thereof, and may alternatively be used in a reversed manner that the belt is turned inside out such that the original back face thereof is used as an inner peripheral side.
In such a reversed position of the belt, if the cord is not positioned at the substantially middle portion of the belt along the belt thickness, the relative position of the cord to the pulley when the belt is wound around the pulley will be changed between the normal position (see FIG. 5a) and the reversed position (see FIG. 5b). This increases a difference between an effective pulley diameter when the bottom-face-side ribs of the V-ribbed belt are faced on the inner peripheral side thereof and an effective pulley diameter when the back-face-side ribs thereof are faced on the inner peripheral side thereof, resulting in an increased departure of a speed ratio of revolution transmitted between the pulleys of the effective pulley diameters from that of revolution transmitted between pulleys of design diameters. In addition, the belt length will also be changed between the bottom and back faces. This needs for a large movement allowance of the pulley movable for tensioning the belt and therefore makes it difficult to provide a compact design of a belt power transmission device.
Particularly for a belt in which the bottom-face-side and back-face-side ribs are arranged at equal pitches so as to be used arbitrarily in either of its normal and reversed positions, an influence derived from the departure from the speed ratio as designed becomes a serious problem. It causes, for example, lack of capacity of an engine auxiliary (a decreased amount of power generation or a decreased hydraulic pressure), deterioration in shaft life due to abnormal increases in rotational speed, and so on.
Therefore, the present invention has its object of suppressing a discrepancy between respective speed ratios of revolution transmitted from one pulley to the other in the normal and reversed positions of a V-ribbed belt having ribs of approximately trapezoidal section as described above on both the bottom and back faces by appropriately positioning a cord of the belt.
To attain the above object, the present invention provides for positioning the center of the cord substantially in the middle of the belt thickness.
More specifically, the present invention is directed to a V-ribbed belt in which a cord is embedded generally in a helical arrangement to form a row along the belt width and a plurality of ribs of approximately trapezoidal section are formed on each of bottom and back faces of the belt and in corresponding positional relation between the bottom and back faces to extend in parallel with each other along the belt length.
In the belt, the ribs in both the bottom and back faces are made of rubber of identical characteristic and formed in approximately identical size and shape, and the cord is disposed substantially in the middle of the belt thickness.
With this arrangement, since the cord is disposed substantially in the middle of the belt thickness, the relative position of the cord to a pulley when the belt is wound around the pulley in each of the normal and reversed positions is identical. This suppresses a discrepancy between the speed ratios of one pulley to the other in both the positions of the belt.
A difference between a belt length when the ribs in the bottom face of the belt are faced on an inner peripheral side thereof and a belt length when the ribs in the back face thereof are faced on the inner peripheral side thereof is preferably 2 mm or less. With this arrangement, there can be obtained a desirable range of positions of the cord to be disposed substantially in the middle of the belt thickness. In other words, if the difference between both the belt lengths is over 2 mm, an effect of suppressing a discrepancy in speed ratio between the normal and reversed positions of the belt is insufficient. Therefore, the difference between both the belt lengths is set at 2 mm or less.
A difference between a distance from the center of the cord to a top end of each of the ribs in the bottom face and a distance from the center of the cord to a top end of each of the corresponding ribs in the back face may be 0.3 mm or less on the average over the entire circumference of the belt. Also in this case, there can be obtained a desirable range of positions of the cord to be disposed substantially in the middle of the belt thickness. In other words, if the difference between both the distances is over 0.3 mm on the average over the entire circumference of the belt, the effect of suppressing the discrepancy in speed ratio between the normal and reversed positions of the belt is insufficient. Therefore, the difference between both the distances is set at 0.3 mm or less.
A top of the rib may include a planar top surface along the belt width and a pair of right and left corners with one end continuing to a left or right end of the top surface and the other end continuing to a side face of the rib, both the corners may be formed in respective arcuate sections having different centers of curvature positioned bilaterally with respect to the centerline of the rib, and when the height of the rib is hr and the curvature radius of the corner of the rib is R, the curvature radius R may be set to meet the following formula: 0.17 hrxe2x89xa6R less than 0.5 hr.
Alternatively, a top of the rib may include a planar top surface along the belt width and a pair of right and left corners with one end continuing to a left or right end of the top surface and the other end continuing to a side face of the rib, both the corners may be formed in respective arcuate sections having different centers of curvature positioned bilaterally with respect to the centerline of the rib, and when the pitch of the ribs is p and the curvature radius of the corner of the rib is R, the curvature radius R may be set to meet the following
formula: 0.14 pxe2x89xa6R less than 0.35 p.
With these arrangements, only the right and left corners of each rib of the belt located on both sides of the planar top surface of the rib are formed in respective arcuate sections having different centers of curvature positioned bilaterally with respect to the centerline of the rib, and the curvature radius R of the corner falls into the range of 0.17 hrxe2x89xa6R less than 0.5 hr or the range of 0.14 pxe2x89xa6R less than 0.35 p. Accordingly, when the belt is in a bent position, it can be effectively prevented that stress is concentrated to the corners of each rib located in the outer periphery of the belt. This improves crack-proof performance of the belt in its bent position and keeps it constant, thereby extending the belt life. In addition, large areas of the rib side faces can be kept into contact with the pulley groove when the belt is fitted into the pulley groove. This ensures an improved power transmission performance of the belt.
If the curvature radius R of the rib corner is less than 0.17 hr or less than 0.14 p, the crack-proof performance of the belt will be insufficient. On the other hand, if the curvature radius R of the rib corner is not less than 0.5 hr or not less than 0.35 p, not only the crack-proof performance of the belt will be insufficient but also the rate of slip thereof will be increased because the length of the contact portion of the rib in contact under pressure with the pulley groove surface to contribute to power transmission is decreased. Therefore, when the curvature radius R of the rib corner is set to fall into the range of 0.17 hrxe2x89xa6R less than 0.5 hr or the range of 0.14 pxe2x89xa6R less than 0.35 p, the belt can improve its crack-proof performance at the rib corner located at the top of the rib while improving its power transmission performance by increasing the area of the contact portion of the rib contributing to power transmission.
Furthermore, the present invention is directed to a method for fabricating a V-ribbed belt in which a cord is embedded generally in a helical arrangement to form a row along the belt width and a plurality of ribs of approximately trapezoidal section are formed on each of bottom and back faces of the belt and in corresponding positional relation between the bottom and back faces to extend in parallel with each other along the belt length. This method includes the steps of grinding a bottom face of a flat belt including a cord embedded therein generally in a helical arrangement to form a row along the belt width until a thickness d1 of the belt after ground satisfies the following formula:
d1=hr+xcex4c+C/2+H/2+(L1xe2x88x92L2)/(2xcfx80)xe2x80x83xe2x80x83{circle around (1)}
where hr is a set height of the rib, xcex4c is a set thickness from an end of the cord adjacent the rib to a bottom of the rib, C is the diameter of the cord, H is a set thickness of the V-ribbed belt, L1 is a bottom-face-side inner length of the belt when the bottom face of the belt is faced on an inner peripheral side thereof, and L2 is a back-face-side inner length of the belt when the back face of the belt is faced on the inner peripheral side thereof, and grinding a back face of the ground flat belt until a thickness d2 of the belt after ground satisfies the following formula:
d2=(hr+xcex4c+C/2)xc3x972xe2x80x83xe2x80x83{circle around (2)}
According to this method, when the V-ribbed belt is fabricated by grinding both faces of the flat belt to form ribs therein, both the faces of the flat belt can be ground such that the cord is positioned substantially in the middle of the thickness of the V-ribbed belt even if the position of the cord embedded in the flat belt is unknown. Accordingly, a V-ribbed belt having a cord positioned substantially in the middle of the belt thickness can be readily obtained.
In the above case, in obtaining the bottom-face-side inner length L1 and the back-face-side inner length L2 of the belt, the bottom-face-side inner length L1 of the belt is preferably obtained with the flat belt yet to be ground wound at the bottom face thereof between measuring pulleys formed of a pair of flat pulleys having equal diameters according to the following formula:
L1=(2xc3x97CD1+Kxcfx80)/xcex1xe2x80x83xe2x80x83{circle around (3)}
where CD1 is the center distance between the measuring pulleys, K is the outer diameter of the measuring pulley, and xcex1 is the coefficient of extension of the flat belt under a load of the measuring pulley, and then the back-face-side inner length L2 of the belt is preferably obtained with the flat belt wound at the back face thereof between the measuring pulleys according to the following formula:
L2=(2xc3x97CD2+Kxcfx80)/xcex1xe2x80x83xe2x80x83{circle around (4)}
where CD2 is the center distance between the measuring pulleys.
Thus, by measuring the center distances CD1 and CD2 between the measuring pulleys, the bottom-face-side and back-face-side inner lengths L1 and L2 of the belt can be readily obtained.